Seatbelt pretensioning retractor assembly including a pretensioner rod

ABSTRACT

A seatbelt pretensioning retractor assembly for use in a vehicle is provided. The seatbelt pretensioning retractor assembly includes a housing adapted for being mounted to a frame and having an interior cavity, a pretensioner tube having an arcuate and curved shape, a sprocket rotatably mounted to the housing and fixedly coupled to a spindle, a driving element as a pretensioner rod disposed within the tube and a guide plate having a guide portion and a rod guide. The pretensioner rod extends in a longitudinal direction from a proximal end portion to a distal end portion. In addition, the pretensioner rod forms a recessed portion or a recessed section to facilitate engaging with the sprocket during pretensioning.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 toU.S. patent application Ser. No. 15/461,534, filed Mar. 17, 2017, thecontents of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The technical field relates generally to seatbelt restraint devices forrestraining an occupant of a vehicle, and more particularly, to devicesfor pretensioning a seatbelt.

BACKGROUND

Seatbelt restraint systems for restraining an occupant in a vehicle seatplay an important role in reducing occupant injury in vehicle crashsituations. Seatbelt restraint systems of the conventional so-called“3-point” variety commonly have a lap belt section extending across theseat occupant's pelvis and a shoulder belt section crossing the uppertorso, which are fastened together or are formed by a continuous lengthof seatbelt webbing. The lap and shoulder belt sections are connected tothe vehicle structure by anchorages. A belt retractor is typicallyprovided to store belt webbing and may further act to manage belttension loads in a crash situation. Seatbelt restraint systems which aremanually deployed by the occupant (so-called “active” types) alsotypically include a buckle attached to the vehicle body structure by ananchorage. A latch plate attached to the belt webbing is received by thebuckle to allow the belt system to be fastened for enabling restraint,and unfastened to allow entrance and egress from the vehicle. Seatbeltsystems, when deployed, effectively restrain the occupant during acollision.

OEM vehicle manufacturers often provide seatbelt restraint systems withpretensioning devices, which tension the seatbelt either during animpact of the vehicle or even prior to impact (also known as a“pre-pretensioner”) to enhance occupant restraint performance. Thepretensioner takes out slack in the webbing and permits the beltrestraint system to couple with the occupant early in the crashsequence. One type of pretensioner acts on the webbing retractor totension the belt. Various designs of retractor pretensioners presentlyexist, including a type known as a roto-pretensioner that incorporates agas generator for generating a pyrotechnic charge. Examples of suchroto-pretensioners are described in U.S. Pat. No. 5,881,962, filed Apr.11, 1995, U.S. Patent Application Publication No. 2006/0243843, filedApr. 27, 2005, U.S. Patent Application Publication No. 2012/0006925,filed Jul. 6, 2010, and U.S. Pat. No. 7,988,084, filed Aug. 2, 2011,which are commonly owned by the assignee of the present application andare hereby incorporated by reference in their entirety for all purposes.Generally, ignition of the pyrotechnic charge or other combustiblematerial creates gas pressure in a chamber having a piston to impartmotion upon a driving element such as a piston, rack and pinion, orseries of balls disposed in a pretensioner tube, which engage with andwind a retractor spool sprocket to retract the webbing.

One issue with pretensioners using a series of metallic balls is theweight of the series of balls required for a full pretensioning stroke,as well as the corresponding cost of supplying multiple metallic ballswith strict tolerances. Further, for pretensioners using a series ofmetallic balls, or rack and pinion based systems, is the need for asynchronizing or clutch feature to ensure that the series of balls orpinion sufficiently engage the retractor spool sprocket.

Another issue with pretensioners is known as a low resistance condition,where the driving elements will reach an end of stroke withoutexperience substantial resistance. This can occur if there is excessiveslack in the seatbelt webbing. In these cases, the low resistanceresults in a lower amount of backpressure from the driving elements. Thebackpressure is produced by the engagement between the driving elementsand the sprocket, so lower backpressure reduces the pressure on asealing element that trails the driving elements. Reduced pressure onthe sealing elements reduces the amount that the sealing element iscompressed circumferentially. Reduced sealing ability can cause gas toleak from the tube around the series of balls.

A further issue with pretensioners is the need to maintain the retractorand the seatbelt webbing in a locked condition at the end of thepretensioning stroke. When the retractor spool does not remain locked,payback can occur which allows the seatbelt to unspool and reintroduceslack in the seatbelt. One method for maintaining the locked positionincludes maintaining pressure from the gas generator beyond the amountneeded for the pretensioning stroke. However, this adds weight and cost.

SUMMARY

Seatbelt pretensioning retractor assemblies for use in a passengervehicle are provided herein. In an exemplary embodiment, a drivingelement in the form of an elongated rod for the motor vehicle seat beltpretensioner having a sprocket, a housing, a guide plate and apretensioner tube includes a distal end portion disposed towards thesprocket and a proximal end portion disposed opposite the distal endportion. The driving element is configured to extend in a longitudinaldirection from the proximal end portion to the distal end portion. Thedriving element further includes a recessed portion defining a recessthat extends generally in a longitudinal direction along an outsidesurface on a first side of the driving element, and recessed sectionextending along the outside surface in the longitudinal direction on asecond side opposite the first side. Furthermore, the second side havingthe recessed section includes a non-recessed section in the distal endportion of the driving element, and the driving element includes a firstportion formed by the recessed section defining a first length along thelongitudinal direction and a second portion formed by the non-recessedsection defining a second length along the longitudinal direction. Thefirst length is greater than the second length.

According to a further aspect of the exemplary embodiment, the firstlength is between 60 mm and 150 mm and the second length is between 10mm and 25 mm. The second length is preferably about 20 mm. A secondcross-sectional area in the non-recessed section is larger than a firstcross-sectional area in the recessed section for reducing an amount ofstripping of the driving element which can occur at first contact withthe sprocket in an actuated state of the pretensioner. The recessessection on the second side of the driving element forms a substantiallyflat surface. Furthermore, the recessed section is sized that athickness of the driving element along a first axis defined normal tothe flat surface of the recessed section is less than a width of thedriving element along a second axis defined perpendicular to the firstaxis to facilitate bending of the driving element in directions of thefirst axis during translation in the longitudinal direction. The drivingelement is made from a polymer material.

In another exemplary embodiment, the driving element includes a distalportion having a distal-most end portion adjacent to an exit of thepretensioner tube when the pretensioner is in a normal non-actuatedstate, and a proximal end portion disposed opposite the distal endportion. The distal-most end portion includes a first chamfer disposedon a first side of the driving element and a second chamfer disposed ona second side opposite the first side of the driving element, and thefirst chamfer and the second chamfer taper inwardly along a length ofthe distal-most end portion in the longitudinal direction.

According to a further aspect of the exemplary embodiment, the firstchamfer is configured to reduce a force required to advance the drivingelement in the pretensioner tube when the driving element is installedinto the pretensioner tube. Each of the first chamfer and the secondchamfer is formed as a curved shape or a flat shape. The first chamferallows the distal-most end portion of the driving element to past aprojection near the exit of the pretensioner tube in an actuated stateof the pretensioner. The second chamfer is configured to facilitateengagement of the driving element with the sprocket duringpretensioning.

In another exemplary embodiment, the driving element includes a distalend portion, a proximal end portion, and a post formed in the proximalend portion. The post extends proximally from an end surface of thedriving element defined perpendicular to the longitudinal direction. Thepost includes one or more ribs extending outwardly from the postsubstantially parallel to the longitudinal direction for coupling astopper to the driving element. The one or more ribs formed on the postfacilitate retaining the stopper to the driving element by aninterference fit. The stopper includes a blind hole for receiving thepost of the driving element.

Further objects, features, and advantages of the invention will becomeapparent to those skilled in the art to which the present inventionrelates from consideration of the following description and the appendedclaims, taken in conjunction with the accompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 is perspective view of an occupant restraint system in accordancewith an exemplary embodiment;

FIG. 2 is a perspective view of the occupant restraint system withvarious components removed to show a seatbelt pretensioning retractorassembly in accordance with an exemplary embodiment;

FIG. 3 is a perspective view of the seatbelt pretensioning retractorassembly in accordance with an exemplary embodiment;

FIG. 4 is a cut-away view of the seatbelt pretensioning retractorassembly illustrating a tube, a polymer rod, and a sprocket in anon-actuated position in accordance with an exemplary embodiment;

FIG. 5 is a side cut-away view of the polymer rod and a stopper inaccordance with an exemplary embodiment;

FIG. 6 is a plan view of the sprocket having a plurality of vanes inaccordance with an exemplary embodiment;

FIG. 7 is a side view of the sprocket in accordance with an exemplaryembodiment;

FIG. 8 is a partial view of the sprocket illustrating the shape of thevanes in accordance with an exemplary embodiment;

FIG. 9 is an exploded, perspective top view of the polymer rod, thestopper, and a seal in accordance with an exemplary embodiment;

FIG. 10A is a partial enlarged view of the polymer rod depicted in FIG.9 in accordance with an exemplary embodiment;

FIG. 10B is a partial enlarged view of the polymer rod depicted in FIG.9 in accordance with an exemplary embodiment;

FIG. 11 is a partial enlarged view of the polymer rod depicted in FIG. 9in accordance with an exemplary embodiment;

FIG. 12 is a partial bottom view of the polymer rod in accordance withan exemplary embodiment;

FIG. 13 is a partial cross-sectional view of the polymer rod in a firstposition in the tube during installation in accordance with an exemplaryembodiment;

FIG. 14 is a partial cross-sectional view of the polymer rod in a fullyinstalled position in the tube in accordance with an exemplaryembodiment;

FIG. 15 is a cross-sectional view of the seatbelt pretensioningretractor assembly in an actuated position in accordance with anexemplary embodiment;

FIGS. 16A-16E are cross-sectional views of various embodiments of thepolymer rod illustrated in FIG. 5 along line 16-16;

FIG. 17 is a cross-sectional view of an embodiment of a constrictionportion in the tube;

FIG. 18 is a cross-sectional view of another embodiment of theconstriction portion;

FIG. 19 is a cross-sectional view of another embodiment of theconstriction portion;

FIG. 20 is a side cut-away view of the polymer rod and the stopper inaccordance with another exemplary embodiment;

FIG. 21 is a plan view of the polymer rod and the stopper depicted inFIG. 20; and

FIGS. 22A-22C are cross-sectional views of the polymer rod depicted inFIG. 21.

It should be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

DETAILED DESCRIPTION OF THE INVENTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure or its application or uses.

Referring now to the drawings, FIG. 1 shows a vehicle seat 9 and aseatbelt assembly 12 in accordance with an exemplary embodiment. Theseatbelt assembly 12 includes a seatbelt webbing 14 having a shoulderbelt portion 16 extending from an upper guide loop or anchorage 18 to alatch plate 20 and a lap belt portion 22 extending from the latch plate20 to an anchorage 24. The latch plate 20 can include a loop portion 26through which the seatbelt webbing 14 extends. The latch plate 20 isable to be inserted into a seatbelt buckle 28 to lock and unlock theseatbelt assembly 12. A seatbelt buckle cable 30, either directly or incooperation with other components, secures the seatbelt buckle 28 to aportion 31 of the vehicle structure (e.g., vehicle frame). It will beappreciated that other manners of attaching the seatbelt webbing 14 tovehicle could also be used, including variations on the latch plate 20and the seatbelt buckle 28 and their attachments to the seatbelt webbing14 and associated vehicle structure.

The seatbelt webbing 14 is able to pay-out from a seatbelt pretensioningretractor assembly or retractor assembly 32 (shown in FIGS. 2 and 3),which is located within the vehicle seat 9 (in an integrated structuralseat design) or is coupled structurally to the vehicle body, so that theeffective length of the seatbelt webbing 14 is adjustable. When thelatch plate 20 has been fastened to the seatbelt buckle 28, the seatbeltassembly 12 defines a three-point restraint between the anchorage 18,the latch plate 20, and the anchorage 24. Any other suitableconfigurations, such as alternative locations for the retractor assembly32, the latch plate 20, and the anchorage 24, may be used with thepresent invention.

Now with reference to FIG. 2, an isometric view of the seatbelt assembly12 is illustrated disassociated from the motor vehicle and showing theretractor assembly 32 in accordance with an exemplary embodiment. Theretractor assembly 32 includes a spool assembly 34 and a gas generator36 mounted to a common frame 38. The spool assembly 34 is connected withand stows the seatbelt webbing 14 of the shoulder belt portion 16,whereas the end of the lap belt portion 22 of the seatbelt webbing 14 isfixedly engaged with the anchorage point, for example, the frame 38 oranother portion of the motor vehicle such as the seat 9 (shown inFIG. 1) or floor pan.

Referring also to FIG. 3, the spool assembly 34 includes a belt spool 40that engages the shoulder belt portion 16 of the seatbelt webbing 14 androtates to wind-up or pay-out the seatbelt webbing 14. A torsional“clock” or “motor” type spring is carried within a spring end cap 42 androtationally biases the belt spool 40 to retract the seatbelt webbing14. The spool assembly 34 may further incorporate other spool controlmechanisms that are known in accordance with the prior art, includingpretensioners, inertia and webbing sensitive locking devices, torsionbar load limiters, or other belt control devices. “Spool controlsystems” referred to in this specification may include any system thatcontrols the rotational movement of a webbing spool, thus controllingthe extraction and retraction of seatbelt webbing. One such spoolcontrol system is a motor-assisted retractor. Spool locking devicestypically incorporate an inertia sensitive element, such as a rollingball or pendulum, and cause a sprocket of the spool to be engaged toprevent further withdrawing of the seatbelt webbing 14 from the beltspool 40. Webbing sensitive locking devices sense rapid pay-out ofseatbelt webbing 14 to lock the retractor assembly 32. Variouselectronic sensing mechanisms that detect the withdrawal of seatbeltwebbing 14 and/or the connection of the latch plate 20 to the seatbeltbuckle 28 may also be incorporated into the retractor assembly 32.

During normal operation of the vehicle, the retractor assembly 32 allowspay-out of seatbelt webbing 14 to give the occupant a certain amount offreedom of movement. However, if an impact or a potential impactsituation is detected, the retractor assembly 32 is locked to preventpay-out and to secure the occupant in the seat 9. For example, if thevehicle decelerates at a predetermined rate or if the brakes areactuated with a predetermined force, then the retractor assembly 32 islocked. Due in part to the free pay-out of the seatbelt webbing 14, theseatbelt assembly 12 often develops slack during normal use.

FIG. 4 provides a cut-away illustration of a pretensioner system 44 inaccordance with an exemplary embodiment. Referring to FIGS. 3-4, inparticular, the retractor assembly 32 further incorporates thepretensioner system 44 operatively connected to the spool assembly 34and operable to rotate the belt spool 40 for pretensioning. As known tothose of skill in the art, a retractor pretensioner winds seatbeltwebbing into a more taught condition against the occupant at the initialstages of a detected vehicle impact. This is provided to reduce forwardmotion or excursion of the occupant in response to the decelerationforces of a vehicle impact or rollover.

The pretensioner system 44 includes a pretensioner tube 52 incommunication with the gas generator 36. The gas generator 36 is used toprovide expanding gas in response to a firing signal. As is known in theart, for example, the vehicle includes a sensor array sending a signalindicative of an emergency event such as an impact event, crash, orrollover. The vehicle sensor may be a specific impact sensor, or may bea traditional vehicle sensor (e.g. a longitudinal or lateralacceleration sensor) or otherwise part of a control system having asuite of multiple sensors. Any other impact sensor that is or will beknown to those skilled in the art may also be readily employed inconjunction with the seatbelt assembly 12 of present invention. Anelectronic control unit such as a central processing unit (CPU) or othercontroller receives a signal and controls the seatbelt assembly 12 torespond by tightening the seatbelt webbing 14 of the vehicle (e.g. viaactivation of a pretensioner).

As will be discussed in further detail below, the pretensioner tube 52has a pretensioner rod 53, e.g., a polymer rod or a plasticallydeformable polymer rod, disposed therein that has an elongate shape andis flexible within the tube 52. More specifically and as will bediscussed in further detail below, the pretensioner rod 53, whendisposed outside of the pretensioner tube 52 prior to insertion therein,has a generally straight shape, and when inserted into the tube 52 itwill bend and flex in accordance with the tortuous shape of the tube 52.

Turning now to the retractor assembly 32, the retractor assembly 32includes the spool assembly 34 mounted to the common frame 38, asdescribed above. More particularly, the spool assembly 34 will rotaterelative to the common frame 38 to wind the seatbelt webbing 14 attachedto the spool assembly 34. The common frame 38 includes a housing 54 forhousing the components of the pretensioner system 44.

The spool assembly 34 includes a sprocket 56 that is disposed within thehousing 54. The sprocket 56 is attached to the belt spool 40. Rotationof the sprocket 56 will cause the attached belt spool 40 to rotate towind the seatbelt webbing 14 that is attached to the belt spool 40.

FIG. 5 provides a side cut-away illustration of the pretensioner rod 53and a stopper 55 in accordance with an exemplary embodiment. FIGS.16A-16E provide cross-sectional views of various embodiments of thepretensioner rod 53 illustrated in FIG. 5 along line 16-16. As shown inFIGS. 5 and 16A-16E, the pretensioner rod 53 has a generally circularcross-section in one form. In another approach, the pretensioner rod 53could have a non-circular cross-section, such as a generally rectangularcross-section, generally triangular cross-section, or other polygonalcross-section that allows the pretensioner rod 53 to be inserted intothe pretensioner tube 52 (shown in FIG. 4) during assembly of thepretensioner system 44 and adapt to the tortuous shape of thepretensioner tube 52 when inserted. For purposes of discussion, thepretensioner rod 53 will be discussed as having a generally circularcross-section

As illustrated and as discussed above, the pretensioner rod 53, whendisposed outside of the pretensioner tube 52, has a generally straightshape and extends in a longitudinal direction 200 from a proximal endportion 202 to a distal end portion 204. The proximal end portion 202 isdisposed towards the gas generator 36 (shown in FIG. 4) when thepretensioner rod 53 is installed within the pretensioner system 44. Inan exemplary embodiment, the pretensioner tube 52 has a cross-sectionthat varies along its length to define a non-recessed portion 206 and arecessed portion 208 that defines a recess 210. In one example and asillustrated in FIGS. 5 and 16D, the recess 210 is configured as a groove212, e.g., U-shaped groove with sidewalls 214 a. In another example andas illustrated in FIG. 16A, the recessed portion 208 has a recessedruled surface 214 b, e.g., substantially flat surface, which defines therecess 210. Non-limiting alternative examples of different forms of therecess 210 are illustrated in FIGS. 16B-16C and 16E.

Referring to FIGS. 5 and 9, in an exemplary embodiment, the recessedportion 208 extends along a majority of the overall length of thepretensioner rod 53 from the proximal end portion 202 to and includingthe distal end portion 204. In one example, the recessed portion 208extends to a distal-most end portion 209 of the distal end portion 204of the pretensioner rod 53. As illustrated, the proximal end portion 202includes the non-recessed portion 206 in which the recess 210 terminatesat a distal-most section of the non-recessed portion 206. As will bediscussed in further detail below and as illustrated in FIGS. 5 and16A-16E, the non-recessed portion 206 of the pretensioner rod 53 has adiameter, cross-sectional dimension, and/or perimeter greater than therecessed portion 208. In an exemplary embodiment, the non-recessedportion 206 has a length L1 of from about 15 to about 25 mm, such asabout 20 mm and a width W1 of from about 4 to about 7 mm, and therecessed portion 208 has a length L2 of from about 60 to about 145 mmand a thickness W2 of from about 2.5 to about 8.0 mm.

Referring to FIGS. 4-5, 9, and 10B, in an exemplary embodiment and aswill be discussed in further detail below, the pretensioner rod 53 has aretention feature 211 that is disposed at or proximate the distal endportion 204 for helping to retain and/or position the pretensioner rod53 in the tube 52 during assembly of the pretensioner system 44. In oneexample, the retention feature 211 is a negative feature 213, e.g., arecess, notch, or hook, which is aligned with or parallel to the recess210 and extending downwardly into the recessed portion 208.Alternatively, the pretensioner rod 53 may not include a retentionfeature 210 as illustrated in FIG. 10A.

Referring to FIGS. 5, 9, and 11, in an exemplary embodiment, thepretensioner rod 53 also includes a positive feature 216, e.g., nub orpost, extending proximally from the proximal end portion 202. Thestopper 55 has a negative feature 218 formed therein that receives thepositive feature 216 to couple the stopper 55 to the proximal endportion 202 of the pretensioner rod 53. In one example, the negativefeature 218 and the positive feature 216 are sized such that the stopper55 is compression fit, e.g., interference fit, onto the positive feature216 to fixedly couple the stopper 55 to the pretensioner rod 53. In anexemplary embodiment and as illustrated in FIG. 11, the positive feature216 includes one or more ribs 217 extending outwardly from the positivefeature substantially parallel to the longitudinal direction 200 tofacilitate retaining the stopper 55 to the pretensioner rod 53. Otherforms of coupling and/or fixing the stopper 55 to the positive feature216 and/or the proximal end portion 202 may be used, such as, forexample, an adhesive, mechanical means, or the like. As illustrated, thenegative feature 218 can be configured as a blind hole 220.Alternatively, the negative feature 218 can be configured as athrough-hole with the positive feature 216 extending partially or fullytherethrough.

In an exemplary embodiment, the pretensioner rod 53 includes a chamferedend wall 219 that is disposed at the proximal end section of therecessed portion 208 that is adjacent to the stopper 55 (shown in FIG.11). The chamfered end wall 219 flares outwardly along a length of theproximal end section in a direction towards the stopper 55. As will bediscussed in further detail below, advantageously the chamfered end wall219 helps slow the pretensioner rod 53 near the end of the pretensioningstroke to reduce impact forces on a projection 120 (shown in FIGS. 4 and15) as the projection 120 moves relative through the recess 210 of thepretensioner rod 53 and engages the tapered structure of the chamferedend wall 219. Without being limited by theory, it is believed that thetapered structure of the chamfered end wall 219 allows the impact forceagainst the projection 120 to be managed (e.g., gradually increased)while effectively reducing the speed of the pretensioner rod 53 over adistance at or near the end of the pretensioning stroke such that themaximum impact force is less than otherwise would have occurred withoutthe chamfered end wall 219 (e.g., versus abrupt impact of a vertical endwall of the recessed portion 208 against the projection 120).

Referring to FIGS. 4-5, 9, 10A-10B, and 12, in an exemplary embodiment,the pretensioner rod 53 may include one or more chamfers 260 a, 260 b atits distal end portion 204 that taper inwardly along a length of thedistal-most end portion 209 in the direction 200. In one example, thepretensioner rod 53 includes the chamfer 260 a that is disposed on asame side of the pretensioner rod 53 as the recess 210. In anotherexample, the pretensioner rod 53 includes the chamfer 260 b that isdisposed on a side of the pretensioner rod 53 opposite the chamfer 260a. In an exemplary embodiment and as will be discussed in further detailbelow, the chamfer 260 a advantageously helps reduce the force requiredto advance the pretensioner rod 53 in the tube 52 to facilitateinstallation of the pretensioner rod 53 in the pretensioner system 44,and independently, the chamfer 260 b advantageously facilitatesengagement of the pretensioner rod 53 with the sprocket 56 duringpretensioning.

With reference to FIGS. 4-5, in an exemplary embodiment, thepretensioner rod 53 is made from a polymer material, which has a reducedweight relative to metallic ball driving elements of otherroto-pretensioners. The particular polymer material can be selected tofit the particular desires of the user. The polymer material ispreferably one that has sufficient flexibility such that it can bend andflex through the pretensioner tube 52 to allow for initial installationas well as in response to actuation by the gas generator 36. The polymermaterial is preferably one that has sufficient pushability in responseto actuation, such that the pretensioner rod 53 will sufficientlytransfer a load to the sprocket 56 of the pretensioner system 44,thereby functioning as a driving element for causing pretensioning.

Further, in an exemplary embodiment, the pretensioner rod 53 is madefrom a polymer material that is plastically deformable. During and afteractuation, the pretensioner rod 53 will be become plastically deformedin response to actuation and contact with other components (e.g., thesprocket 56) of the pretensioner system 44. As will be discussed infurther detail below, this plastic deformation will cause thepretensioner rod 53 to become locked, for example, in the sprocket 56 toprevent or limit payback of the pretensioner rod 53 without beingcompletely dependent on maintained pressure in the system.

In one approach, the pretensioner rod 53 is made from a nylonthermoplastic material. The pretensioner rod 53 could also be made froman aliphatic polyamide thermoplastic material. In another approach, thepretensioner rod 53 could be made from a similar thermoplastic material,such as an acetal material or polypropylene material.

With reference to FIGS. 6-8, the sprocket 56 has a general annular shapethat defines an internal aperture 76. The sprocket 56 further defines aplurality of inward extending teeth 78 that extend into the aperture 76.The teeth 78 are sized and configured to correspond to recesses ordepressions that are defined on an external surface of the belt spool 40(shown in FIG. 3) to connect the sprocket 56 to the belt spool 40 suchthat they are rotationally coupled. In another approach, the internalaperture 76 could include a single tooth or protrusion to rotationallycouple the sprocket 56 to the belt spool 40.

The sprocket 56 includes an annular body portion 80 and a flange portion82 that projects radially from the base of the annular body portion 80.The sprocket 56 further includes a plurality of vanes 84 that eachproject radially from the annular body portion 80 as well aslongitudinally from the flange portion 82, such that vanes 82 extendbetween the annular body portion 80 and the flange portion 82.

The vanes 84 each have a generally triangular shape when viewed from thefront, with a base that tapers into a point 86 as the vane 84 extendsradially outward from the annular body portion 80. The particular widthand pitch of the vanes 84 can be selected as desired. The plurality ofvanes 84 combine to define cavities 88 that are disposed betweenadjacent vanes 84.

In one approach, each of the vanes 84 can have the same size and shapeand be uniformly distributed around the sprocket 56. In anotherapproach, the vanes 84 can have different sizes and/or be spaced atdifferent intervals. Adjusting the size and spacing of the vanes 84 canalter the amount of rotation and/or the rate of the rotation for thesprocket 84 when the pretensioner system 44 is activated. This variablesize and/or spacing is possible due to the actuation by the pretensionerrod 53 rather than by a series of similarly shaped balls. In apretensioner that uses a plurality of ball-shaped driving elements, thesize and spacing is preferably uniform to account for the predeterminedshape and size of the balls.

With reference once again to FIG. 4, the housing 54 further include aguide portion 90. The guide portion 90 is disposed within the housing 54similar to the sprocket 56. More particularly, the guide portion 90 isdisposed opposite an exit 89 of the tube 52, and the sprocket 56 isdisposed between the guide portion 90 and the tube 52. Accordingly, thepretensioner rod 53 exiting the tube 52 will contact the sprocket 56prior to contacting the guide portion 90.

The guide 90 has a generally arcuate landing surface 92 that has aconcave shape toward the exit 89 of the tube 52. In one approach, thearc of the surface 92 has a constant radius. Further, the center pointof the radius of the arc is aligned with the rotational axis of thesprocket 56, such that the radial spacing between the surface 92 and thesprocket 56 is consistent along the length of the surface 92. In anotherapproach, the center point of the radius of the surface 92 could beoffset from sprocket axis, such that the radial spacing between thesurface 92 and the outer diameter of the sprocket 56 will vary atdifferent points along the surface 92.

The surface 92 includes a first end 96 and second end 98. The first end96 is disposed opposite the exit 89 of the tube 52 such that thepretensioner rod 53 would engage the first end 96 prior to the secondend 98 after exiting the tube 52 and passing the sprocket 56.

The housing 54 further defines an overflow cavity 100 that is disposedopposite the guide 90. The overflow cavity 100 is also disposed adjacentthe curvature of the tube 52, and the sprocket 56 is disposed betweenthe guide 90 and the overflow cavity 100. Accordingly, an intermediateportion 101 of the guide 90 is diametrically opposite the overflowcavity 100 across the sprocket 56.

The overflow cavity 100 is sized and configured to allow a portion thepretensioner rod 53 to be received therein during actuation ofpretensioner system 44, if necessary. For example, after thepretensioner rod 53 has exited the tube 52 it will contact the guide 90and be directed in an arcuate path corresponding to the guide 90, suchthat the pretensioner rod 53 is ultimately directed toward the overflowcavity 100. The pretensioner rod 53 can extend into the overflow cavity100, and can further be guided along the curvature of the tube 52 thatis adjacent the overflow cavity 100. However, it will be appreciatedthat the pretensioner rod 53 may not necessarily travel far enoughduring actuation to ultimately reach the overflow cavity 100.

As described above, the retractor assembly 32 includes the gas generator36 that provides expanding gas in response to a firing signal. Theexpanding gas causes an increase in pressure within the tube 52, whichultimately causes the pretensioner rod 53 to be forced away from the gasgenerator 36, through the tube 52, and pass the exit into the sprocket56 for pretensioning.

More particularly, as shown in FIGS. 9 and 15, the pretensioner tube 52includes a piston or seal member 102. The seal member 102 can have agenerally cylindrical shape with a generally cylindrical outer surface,in one approach. In another approach, the seal member 102 can have aspherical shape with a spherical outer surface, or another appropriateshape and outer surface for sealing. The seal member 102 is slidablydisposed within the tube 52 and is operable to drive the pretensionerrod 53 along an actuating direction or path A. As will be understood bythose of skill in the art, the seal member 102 may be press-fitted orotherwise fitted inside the tube 52.

The seal member 102 defines a proximal end spaced from the gas generator36 so as to define a gas chamber therebetween. The seal member 102defines a distal end directed toward the stopper 55 and the pretensionerrod 53.

The stopper 55 is preferably made from aluminum, but could be made fromanother suitable material of sufficient strength, such as steel, othermetal or metal alloy, or reinforced plastic with the seal member 102being generally softer such that it can provide the described sealingabilities. In an exemplary embodiment, the stopper 55 has an outerperimeter that substantially matches the perimeter of the non-recessedportion 206 of the pretensioner rod 53. The stopper 55 is adjacent thedistal end of the seal member 102 and abuts the non-recessed portion 206of the pretensioner rod 53.

The seal member 102 and the stopper 55 cooperate to transfer the energyfrom the increased pressure in the gas chamber to the pretensioner rod53. The pretensioner rod 53, in order to travel through the tube 52 andflex according to the curvature of the tube 52, is sized slightlysmaller than the width of the tube 52. Thus, without the seal member102, gas from the gas generator 36 would flow past the pretensioner rod53 in the space defined between the pretensioner rod 53 and the tube 52.

The seal member 102 defines a generally elastic structure, and may becomposed of various materials known in the art, such as any suitableplastic or polymer (e.g., polyester, rubber, thermoplastic, or otherelastic or deformable material). Moreover, the seal member 102 may bedie cast, forged, or molded from metal, plastic, or other suitablematerial. In one embodiment, the seal member 102 is formed using atwo-cavity injection molding process. The generally elastic structureallows the shape of the seal member 102 to change slightly in responseto pressure, thereby improving the sealing that it provides.

Referring to FIGS. 4 and 15, in operation, the gas generator 36 producesexpanding gas that pressurizes the gas chamber, thereby enabling theseal member 102 to forcibly drive the pretensioner rod 53 along theactuation path A. As the pretensioner rod 53 is driven through the tube52, it engages the sprocket 56. More particularly, the pretensioner rod53 engages the vanes 84 of the sprocket 56. Engagement of thepretensioner rod 53 with the sprocket 56 as the pretensioner rod 53 isdriven by expanding gas in the direction of arrow A causes the beltspool 40 (shown in FIG. 3) to rotate, which in turn providespretensioning.

Activation of the gas generator 36 enables the seal member 102 to resistgas leakage. As previously mentioned, the seal member 102 is composed ofa relatively elastic material. Therefore, pressurized gas within the gaschamber 108 causes the proximal end of the seal member 102 to expand,which helps prevent gas from escaping past the seal member 102.

In addition, backpressure generated from the pretensioner rod 53 causesthe seal member 102 to expand circumferentially outward due tocompression of the seal member 102 against the stopper 55 and thepretensioner rod 53. The pretensioner rod 53 undergoes resistance as itengages the sprocket 56 during actuation, thereby generatingbackpressure on the stopper 55 and the seal member 102. Thecircumferential expansion of the seal member 102 provides a tightenedseal between the outer surface of the seal member 102 and the inner wallof the pretensioner tube 52. Accordingly, the seal member 102 of thepresent invention is operable to retain a high seal pressure as well asmaintain residual gas pressure within the tube 52.

During pretensioning of the seatbelt there can be a side-effect known aspayback, where the tension on the seatbelt caused by the occupant duringan event triggering pretensioning can rotate the spool in a directionopposite the pretensioning rotation. This rotation is transferred to thesprocket 56 and the driving elements, causing the driving elements totravel in the reverse direction within the tube 52. Payback can becounteracted by maintaining pressure in the tube 52, but this requiresthe gas generator 36 to fire for a longer period and additionalpropellant.

However, in an exemplary embodiment, the pretensioner system 44described herein includes features configured to counteract the abovedescribed payback side-effect as an alternative to or in addition tomaintained gas pressure. As described above, the pretensioner rod 53 ispreferably made of a plastically deformable material, such as a polymer.

During actuation of the pretensioner system 44, the pretensioner rod 53exits the tube 52 and contacts the vanes 84 of the sprocket 56, causingthe sprocket 56 to rotate. As the pretensioner rod 53 continues past thesprocket 56 causing it to rotate, additional ones of vanes 84 willcontact the side of the pretensioner rod 53, causing the pretensionerrod 53 to be compressed and deformed plastically in the area ofinterference between the vanes 84 and the pretensioner rod 53. Thiscompression will also cause the pretensioner rod 53 to be compressedagainst the guide 90, creating a press-fit configuration of thepretensioner rod 53 between the sprocket 56 and the guide 90.

Additionally, the pretensioner rod 53 and guide 90 may be made frommaterials that will weld together at the end of the pretensioningstroke. For example, the materials of the pretensioner rod 53 and guide90 may be selected such that heat generated from the friction betweenthe pretensioner rod 53 and the guide 90 will cause the pretensioner rod53 and guide 90 to become welded together along an interface W where theguide 90 and rod 53 contact each other. Once the pretensioner rod 53 andguide 90 are welded together, the pretensioner rod 53 will become lockedand prevented or substantially limited from traveling back into the tube52. The plastic deformation of the pretensioner rod 53 caused by thevanes 84 will prevent or substantially limit the sprocket 56 fromrotating in the opposite direction, thereby preventing or substantiallylimiting payback.

The welding results from the relatively high heat and pressure generatedby the system during actuation. For the pretensioner rod 53 and guide 90to weld, the materials used for each are preferably in the same family.For example, if the guide 90 is nylon, then the pretensioner rod 53 ispreferably nylon. Similarly, if the guide 90 is acetal, then thepretensioner rod 53 is preferably acetal. If the guide 90 ispolypropylene, then the pretensioner rod 53 is polypropylene. It will beappreciated that other materials that will weld together under high heatand pressure could also be used. Moreover, it will be appreciated thatsome different types of materials can weld together.

Another side-effect that can occur during pretensioning is known as alow-resistance condition. This can occur when there is a relativelylarge portion of the seatbelt webbing that can be taken up or wound bythe spool in response to actuating the pretensioner. For example, ifthere was extra slack in the seatbelt, this slack would be taken up andwound with lower resistance because it would not be acting on theoccupant until the slack was taken up. In a low resistance condition,the backpressure of the driving elements is reduced. Reducedbackpressure can result in a reduced ability of the sealing element toexpand circumferentially against the inner wall surface of the tube inresponse to the backpressure. This can occur for any type of piston orseal that is configured to expand circumferentially in response tobackpressure as part of its sealing process.

With reference to FIGS. 4, 15, and 17-19, to address the side-effect ofa low-resistance condition of the pretensioner system 44, in anexemplary embodiment, the tube 52 includes the projection 120 as brieflydiscussed above. The projection 120 extends within the tube 52 near anend of the tube 52 to form a constriction portion 130 proximate to wherethe pretensioner rod 53 exits, thereby reducing the cross-sectional areaof the tube 52 in a distinct location. That is, the opening diameter,width, or dimension(s) of the constriction portion 130 is smaller thanthe diameter, width, or dimension(s) of adjacent portions of the tube 52portions, such as, for example, the portions of the tube 52 that areupstream from the constriction portion 130.

As will be discussed in further detail below, the recess 210 is alignedwith the projection 120 along the actuating direction or path A suchthat during actuation (e.g., pretensioning), the recessed portion 208 ofthe pretensioner rod 53 is not obstructed by the projection 120.Moreover, the constriction portion 130 is sized such that there isenough space that at least the recessed portion 208 of the pretensionerrod 53 can travel past the constriction portion 130, but the stopper 55and seal member 102 will be blocked from travelling past theconstriction portion 130. When the stopper 55 and the seal member 102are blocked from advancing past the constriction portion 130, theconstriction portion 130 provides additional backpressure. Accordingly,the seal member 102 will circumferentially or radially expand inresponse to this backpressure, thereby providing an improved seal inlow-resistance conditions. This improved sealing will prevent or limitthe potential for gas to escape from the tube in low-resistanceconditions.

The projection 120 defining the constriction portion 130 can be formedin a variety of ways and have a variety of shapes while providing theabove described functionality. In one approach, shown in FIGS. 4 and 15,the projection 120 is in the form of a crimp or bump 132 in the tube 52,such that the sidewall of the tube 52 maintains substantially the samethickness. The bump 132 is integrally formed with the tube as amonolithic structure, at least in the area of the constriction portion130. Put another way, the bump 132 is not a separate component ormaterial attached to the tube 52. The bump 132 protrudes into the tube52, and has a corresponding depression on an outer surface of the tube52. The bump 132 has a convex shape within the tube 52 and thedepression has a corresponding concave shape facing opposite the convexshape.

In another approach, shown in FIG. 17, the projection 120 could be inthe form of an increased thickness portion 138 that is integral with thetube 52. This is similar to the bump 132, but does not have acorresponding depression on outer surface of the tube 52.

In another approach, shown in FIG. 18, the projection 120 is in the forma separate piece or crescent 136 that is attached to the tube 52 withinthe tube 52. The crescent 136 a, 136 b, or 136 c can be attached viawelding, adhesive, mechanical fasteners, or the like.

In another approach, shown in FIG. 19, the projection 120 could be inthe form of a plate 140 that is inserted through an opening 142 formedin the sidewall of the tube 52. The plate 140 is removably insertedthrough the opening 142 and secured to the tube 52 via known securementmechanisms. In another approach, the plate 140 can be fixedly securedafter insertion through the opening 142. The use of the plate 140 allowsfor different shapes and sizes and materials to be easily selected andinstalled, if desired.

In the above approaches, the projection 120 with the recess 210 of thepretensioner rod 53 aligned therewith may be disposed along an outboardportion of the tube 52 as illustrated in FIGS. 4 and 15. Advantageously,in an exemplary embodiment, disposition of the projection 120 on theoutboard portion of the tube 52 facilitates directing the distal endportion 204 of the pretensioner rod 53 away from the outboard portion ofthe tube 52 into engagement with the vanes 84 of the sprocket 56,thereby engaging the fully or solid side of the pretensioner rod 53(e.g., sprocket 56 engaging side of the pretensioner rod 53 opposite therecess 210) during actuation. This can help to lock the pretensioner rod53 with the sprocket 56 to prevent or reduce any translation of thepretensioner rod 53 in a direction opposite the actuating direction orpath A. Alternatively, the projection 120 with the recess 210 of thepretensioner rod 53 aligned therewith may be disposed along an inboardportion, or any other side portion, of the tube 52.

As described above, the pretensioner rod 53 has the non-recessed portion206 at its proximal end portion 202, where the non-recessed portion 206has a larger diameter or cross-sectional dimension(s) than the recessedportion 208. In one approach, the non-recessed portion 206 has adiameter or cross-sectional dimension(s) that is larger than the widthor cross-sectional dimension(s) of the tube 52 at the constrictionportion 130. Accordingly, with the non-recessed portion 206 disposedupstream of the constriction portion 130, the constriction portion 130may prevent the non-recessed portion 206 from passing.

In another approach, the non-recessed portion 206 can be smaller thanthe width or cross-sectional dimension(s) of the tube 52 at theconstriction portion 130. With the non-recessed portion 206 being smallenough to pass the constriction portion 130, it can pass beyond theconstriction portion 130.

Referring to FIGS. 4, 12, and 20-22C, in an exemplary embodiment, thepretensioner rod 53 includes a recessed section 250 to furtherfacilitate bending and preventing or minimizing twisting of thepretensioner rod 53 of the pretensioner rod 53 through the tube 52during translation towards the sprocket 56. In particular, on a sideopposite the recess 210 of the pretensioner rod, is the recessed section250 that defines a recess 252 and that extends in the longitudinaldirection 200. In an exemplary embodiment, the cross-section of thepretensioner tube 52 varies along its length to define a non-recessedsection 254 at the distal end portion 204 and the recessed section 250extends from the non-recessed section 254 to, for example, aproximal-most end of the proximal end portion 202 to define the recess252. As such, the recessed section 250 extends along the majority of theoverall length of the pretensioner rod 53. In an exemplary embodiment,it has been found that by not extending the recess 252 through thedistal end portion 204, the distal end portion 204 has a relativelylarger or fuller cross-sectional area for engaging the sprocket 56during translation, thereby reducing the amount of stripping of thepretensioner rod 53 which can occur at first contact with the sprocket56, thereby increasing performance of the pretensioner system 44.Moreover, it has been found that by not having the recess/groove 210extend through the proximal end portion 202, the proximal end portion202 has a relatively larger or fuller cross-sectional area for thestopper 55 to contact to reduce the amount of initial compression on thepretensioner rod 53 during actuation of the gas generator 36, therebyincreasing performance of the pretensioner system 44.

In an exemplary embodiment, the recessed section 250 is sized such thata thickness 256 of the pretensioner rod 53 defined normal to therecessed section 250 is less than a width 258 of the pretensioning roddefined normal to the thickness to facilitate bending and/or preventing,reducing, or minimizing twisting of the pretensioner rod 53 through thepretensioner tube 52 during translation to the sprocket 56. Withoutbeing limited by theory, it is believed that the recessed section 250helps reduce the section modulus across the thickness 256 of thepretensioner rod 53 to allow the pretensioner rod 53 to bend more easilyacross its thickness 256. Further, it is believed that this reduction insection modulus, via reducing the thickness of the rod 53 withoutreducing its width, helps prevent twisting of the pretensioner rod 53and reduce friction as the rod 53 is being advanced through thepretensioner tube 52 during translation, thereby increasing performanceof the pretensioner system 44. Moreover, the recessed section 250facilitates insertion of the pretensioner rod 53 into the pretensionersystem 44 during assembly due to its reduced cross-sectional area. In anexemplary embodiment, the non-recessed section 254 has a length L10 offrom about 15 to about 25 mm, such as about 20 mm; the recessed section250 has a length L20 of from about 60 to about 145 mm; the thickness ofthe pretensioner rod 53 is, for example at the proximal end portion 202,from about 2.5 to about 7 mm; and the width of the pretensioner rod 53is, for example at the proximal end portion 202, from about 4 to about 8mm.

In an exemplary embodiment, when the pretensioner rod 53 is disposedoutside of the pretensioner tube 52, the recessed section 250 has asubstantially flat, planar surface defining the recess 252. In anotherexample, the recessed section 250 has a recessed ruled surface thatdefines the recess 252. Non-limiting alternative forms of the recess 252may also be used.

Referring to FIGS. 4 and 13-14, in an exemplary embodiment and asdiscussed above, during assembly of the pretensioner system 44, thepretensioner rod 53 is inserted into the tube 52 along direction A. Asthe distal end portion 204 of the pretensioner rod 53 advances throughthe tube 52, the distal end portion 204 contacts the projection 120 nearthe exit 89 of the tube 52. The chamfer 260 a at the distal end portion204 helps the pretensioner rod 53 to easily advanced over the proximalsidewall 249 of the projection 120 to allow the retention feature 210 toreleasably engage the projection 120 to retain and/or position thepretensioner rod 53 in the tube 52. In particular, the chamfer 260 ahelps reduce the force required to advance the pretensioner rod 53 byallowing the distal-most end of the distal end portion 204 to glide overand/or past the projection 120 so as to minimize or prevent thepretensioner rod 53 from being obstructed by the projection 120. Asillustrated, the negative feature 213 receives the projection 120.During pretensioning in response to actuation by the gas generator 36,force is applied to the pretensioner rod 53 and the retention feature211 releases, flexes, and/or passes over the projection 120 to allow thepretensioner rod 53 to advance towards the sprocket 56 as discussedabove.

As a person skilled in the art will readily appreciate, the abovedescription is meant as an illustration of the implementation of theprinciples of this invention. This description is not intended to limitthe scope or application of this invention in that the invention issusceptible to modification, variation, and change, without departingfrom the spirit of this invention as defined in the following claims.

While the above description constitutes the preferred embodiment of thepresent invention, it will be appreciated that the invention issusceptible to modification, variation and change without departing fromthe proper scope and fair meaning of the accompanying claims.

What is claimed is:
 1. A driving element for a motor vehicle seat beltpretensioner having a sprocket, a housing, a guide portion and apretensioner tube, the driving element comprising: the driving elementin the form of an elongated rod having a distal end portion disposedtowards the sprocket; a proximal end portion disposed opposite thedistal end portion, the driving element being configured to extend in alongitudinal direction from the proximal end portion to the distal endportion; a recessed portion defining a recess that extends generally ina longitudinal direction along an outside surface on a first side of thedriving element; and a recessed section extending along the outsidesurface in the longitudinal direction on a second side opposite thefirst side, wherein the second side having the recessed section includesa non-recessed section in the distal end portion of the driving element,and wherein the driving element includes a first portion formed by therecessed section defining a first length along the longitudinaldirection and a second portion formed by the non-recessed sectiondefining a second length along the longitudinal direction, and the firstlength is greater than the second length.
 2. The driving element ofclaim 1, wherein the first length is between 60 mm and 150 mm, and thesecond length is between 10 mm and 25 mm.
 3. The driving element ofclaim 2, wherein the second length is about 20 mm.
 4. The drivingelement of claim 1, wherein a second cross-sectional area in thenon-recessed section is larger than a first cross-sectional area in therecessed section for reducing an amount of stripping of the drivingelement which can occur at first contact with the sprocket in anactuated state of the pretensioner.
 5. The driving element of claim 1,wherein the recessed section on the second side of the driving elementforms a substantially flat surface.
 6. The driving element of claim 5,wherein the recessed section is sized such that a thickness of thedriving element along a first axis defined normal to the flat surface ofthe recessed section is less than a width of the driving element along asecond axis defined perpendicular to the first axis to facilitatebending of the driving element in directions of the first axis duringtranslation in the longitudinal direction.
 7. The driving element ofclaim 1, wherein the driving element is made from a polymer material. 8.A driving element for a motor vehicle seat belt pretensioner having asprocket, a housing, a guide portion and a pretensioner tube, thedriving element comprising: the driving element in the form of anelongated rod having a distal end portion disposed towards the sprocket,the distal end portion having a distal-most end portion adjacent to anexit of the pretensioner tube when the pretensioner is in a normalnon-actuated state; and a proximal end portion disposed opposite thedistal end portion, the driving element being configured to extend in alongitudinal direction from the proximal end portion to the distal endportion, wherein the distal-most end portion includes a first chamferdisposed on a first side of the driving element and a second chamferdisposed on a second side opposite the first side of the drivingelement, wherein the first chamfer and the second chamfer taper inwardlyalong a length of the distal-most end portion in the longitudinaldirection, and wherein the first chamfer allows the distal-most endportion of the driving element to past a projection near the exit of thepretensioner tube in an actuated state of the pretensioner.
 9. Thedriving element of claim 8, wherein the first chamfer is configured toreduce a force required to advance the driving element in thepretensioner tube when the driving element is installed into thepretensioner tube.
 10. The driving element of claim 8, wherein each ofthe first chamfer and the second chamfer is formed as a curved shape ora flat shape.
 11. The driving element of claim 8, wherein the secondchamfer is configured to facilitate engagement of the driving elementwith the sprocket during pretensioning.
 12. The driving element of claim8, wherein a recessed portion defining a recess extends generally in thelongitudinal direction along an outside surface on the first side of thedriving element.
 13. The driving element of claim 8, wherein a recessedsection having a substantially flat surface extends generally in thelongitudinal direction along an outside surface on the second side ofthe driving element.
 14. The driving element of claim 8, wherein thedriving element is made from a polymer material.
 15. A driving elementfor a motor vehicle seat belt pretensioner having a sprocket, a housing,a guide portion and a pretensioner tube, the driving element comprising:the driving element in the form of an elongated rod having a distal endportion disposed towards the sprocket; a proximal end portion disposedopposite the distal end portion, the driving element being configured toextend in a longitudinal direction from the proximal end portion to thedistal end portion; and a post formed in the proximal end portion, thepost extending proximally from an end surface of the driving elementdefined perpendicular to the longitudinal direction, wherein the postincludes one or more ribs extending outwardly from the postsubstantially parallel to the longitudinal direction for coupling astopper to the driving element.
 16. The driving element of claim 15,wherein the one or more ribs formed on the post facilitate retaining thestopper to the driving element by an interference fit.
 17. The drivingelement of claim 15, wherein the stopper includes a blind hole forreceiving the post of the driving element.
 18. The driving element ofclaim 15, wherein the driving element is made from a polymer material.